I'm having almost same issue... but from the beginning: I was trying to find a kernel, that has build-in support for the ppp_async. I tried some different Sense-based and AOSP ROMs, but none of them worked. The DUN plugin always stayed in Idle-Mode. So I set up a build environment and compiled the kernel by myself (Andromadus 3.0.57). According to the config file, it already had build-in ppp_async support... but nevermind. I changed the kernel config from "*" to "M" to create the module as .ko file. All this worked fine.
After flashing the kernel, I tested whether the module is loadable by using "insmod /system/lib/modules/ppp_async.ko". It took me some builds to make it loadable, because I had the same issues with vermagic as described in another post here.
After a reboot I tried to connect to my RNS510 Premium in the car, using DUN options "Use kernel module" and "Auto connect". RSAP connection was established fine after some seconds. Then DUN status says "Connecting..." and after a few more seconds, it switches to "Error". I tried to open the browser but I don't have a connection. Also the network stats stay empty.
I checked also the log files, but to tell the truth: I don't have any clue where to look at to find out what is going wrong. I'd be very happy, if you could have a look at it. I can provide you my logcat file by email. To your admin-adress from here? Thank you!